1. Field of the Invention
The present invention relates generally to a method and arrangement for determining coefficients for linear predictive coding (LPC), and more specifically to such an arrangement and method by which the number of calculations for deriving LPC coefficients can be markedly reduced.
2. Description of the Prior Art
As is well known in the art, LPC is a method of analyzing a speech signal and characterizing that signal in terms of coefficients which can be encoded, received and decoded to reproduce a close approximation to the original signal. As one of the methods of analyzing a speech signal using the LPC, a covariance method has been disclosed in U.S. Pat. No. 4,544,919.
Before turning to the present invention it is deemed advantageous to briefly discuss the method for determining LPC coefficients which has been disclosed in the above-mentioned U.S. Patent.
Reference is made to FIG. 1, wherein there is shown a flowchart which characterizes the sequence of operations of the aforesaid prior art.
In FIG. 1, it is assumed that an original speech signal to be treated has been sampled or discreted. After start 10, autocorrelation coefficients are calculated from the sampled speech signal using the following autocorrelation function g(i,k) at step 12. The determination of the autocorrelation coefficients is well known in the art. ##EQU1## where s(n), 0.ltoreq.n.ltoreq.N-1 are samples of the speech signal during a frame, and Np is the order of reflection coefficients.
Merely for the sake of simplifying the discussions, it is assumed that the number of the samples within a frame is 160 (viz., s(0), s(1), s(2), . . ., s(159)) and Np equals 10. Accordingly, equation (1) is given by: ##EQU2## In this instance, the number of elements (viz., g(i,k)) totals 121. The elements g(i,k) are represented in the form of matrix with 11-row and 11-column (viz., 11.times.11 matrix) as indicated below. ##EQU3## At step 14, three types of arrays f, c and b are derived from the autocorrelation function g(i,k) using the following equations (3), (4) and (5). ##EQU4##
It is understood that: ##EQU5## Each of these three matrices F, C and B is 10.times.10 square matrix.
Following this, the value of j (where j indicates a reflection coefficient loop variable) is set to 1 (step 16) and, the j-th reflection coefficient r[j] is determined using conventional techniques.
At step 20, the value of j is checked to see if j=Np. In this instance the answer is "NO" and hence control goes to step 22 wherein the arrays f, c and b are updated.
The value of j is incremented at step 24 and, control goes back to step 18. These operations at steps 18, 20, 22 and 24 are repeated until j=Np. Since it has been assumed that Np=10, the number of the loop (steps 18.fwdarw.20.fwdarw.22.fwdarw.24.fwdarw.18) amounts to 9. Consequently, the number of the updating operations for determining LPC coefficients are:
______________________________________ j = 1 3 .times. 9.sup.2 = 243 j = 2 3 .times. 8.sup.2 = 192 j = 3 3 .times. 7.sup.2 = 147 j = 4 3 .times. 6.sup.2 = 108 j = 5 3 .times. 5.sup.2 = 75 j = 6 3 .times. 4.sup.2 = 48 j = 7 3 .times. 3.sup.2 = 27 j = 8 3 .times. 2.sup.2 = 12 j = 9 3 .times. 1.sup.2 = 3 Total 855 ______________________________________
As discussed above, the above-mentioned known technique has encountered the drawback that a relatively large number of the update operations is required for determining the LPC coefficients. This means that such a large number of addressing operations must be carried out and results in the operation speed being undesirably slowed down.